1. Field of the Invention
This invention relates to a lithium ion secondary battery comprising a positive electrode and a negative electrode facing each other via a separator supporting an electrolyte. More particularly, it relates to a battery structure having improved electrical connections between each of a positive electrode and a negative electrode and a separator, which needs no hard metallic case and allows a battery to take an arbitrary shape, such as a thin shape.
2. Description of the Related Art
There has been an eager demand for reduction in size and weight of portable electronic equipment, and the realization relies heavily on improvement of battery performance. To meet the demand, development and improvement of a variety of batteries have been proceeding. Battery characteristics expected to be improved include increases in voltage, energy density, resistance to high load, freedom of shape, and safety. Out of available batteries, lithium ion batteries are secondary batteries that have achieved the most high voltage, the most high energy density, and the most excellent resistance to high load and will undergo successive improvements.
A lithium ion secondary battery mainly comprises a positive electrode, a negative electrode, and an ion conducting layer interposed between the electrodes. The lithium ion secondary batteries that have been put to practical use employ a positive electrode plate prepared by applying to an aluminum current collector a mixture comprising a powdered active material, such as a lithium-cobalt complex oxide, a powdered electron conductor, and a binder resin; a negative electrode plate prepared by applying to a copper current collector a mixture of a powdered carbonaceous active material and a binder resin; and an ion conducting layer made of a porous film of polyethylene, polypropylene, etc. filled with a nonaqueous solvent containing lithium ions.
FIG. 3 schematically illustrates the cross section of a conventional cylindrical lithium ion secondary battery disclosed, e.g., in JP-A-8-83608 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). In FIG. 3 reference numeral 1 indicates a battery case made of stainless steel, etc. which also serves as a negative electrode terminal, and numeral 2 an electrode body put into the battery case 1. The electrode body 2 is composed of a positive electrode 3, a separator 4, and a negative electrode 5 in a rolled-up form. In order for the electrode body 2 to maintain electrical connections among the positive electrode 3, the separator 4, and the negative electrode 5, it is necessary to apply pressure thereto from outside. This has been achieved by putting the electrode body 2 into the rigid case 1 which is to apply pressure thereby keeping the planar contacts among the above three elements. In the case of rectangular batteries, an external pressing force is imposed to a bundle of strips of an electrode body by, for embodiment, putting the bundle in a rectangular metal case.
That is, a contact between a positive electrode and a negative electrode with a separator therebetween in commercially available lithium ion secondary batteries has been made by using a hard case made of metal, etc. Without such a case, the electrode body would be separated, and the battery characteristics would be deteriorated due to a faulty electrical connection. However, because the case occupies a large proportion in the weight and volume of a battery, it contributes to not only reduction in energy density of the battery but limitation of the battery shape, making it difficult to make a battery of arbitrary shape.
Under such circumstances, development of lithium ion secondary batteries which do not require a hard case has been proceeding, aiming at reductions in weight and size (thickness). The key to development of batteries requiring no hard case is how to maintain an electrical connection between each of a positive electrode and a negative electrode and an ion conducting layer (i.e., separator) interposed therebetween without adding an outer force.
Connecting means requiring no outer force that have been proposed to date include a structure in which a positive electrode and a negative electrode are adhered together with a liquid adhesive composition (electrolyte gel) as disclosed in U.S. Pat. No. 5,460,904 and a structure in which a positive electrode and a negative electrode are formed by binding the respective active material with an electron conducting polymer, and the positive and negative electrodes are adhered together with a polymer electrolyte as disclosed in U.S. Pat. No. 5,437,692.